Continuous ambulatory measurement of core body temperature (CT) can be a critical component of human heat strain assessment during strenuous work (Moran et al 1998; Frank et al 2001). However, while personal physiological monitoring technology has developed to the point where multi-parameter sensor systems can be used to collect data in a variety of settings over extended periods of time, the requisite measurement of core body temperature still remains a challenge.
Medical grade core body temperature measurement using pulmonary arterial blood temperature is only appropriate in a clinical setting. The traditionally accepted laboratory rectal and esophageal probe methods are impractical for ambulatory settings. Other non-invasive methods of estimating core body temperature using external measurements such as axillary or tympanic temperatures have proven unreliable (Lim, Byrne and Lee 2008). Ingestible thermometer pills (e.g., Jonah Pill thermometer, Respironics, Bend, Oreg.) have been used successfully in field settings (e.g. Lee et al 2010), and have been within acceptable limits of agreement (±0.4° C.) and bias (<0.1° C.) when compared to esophageal temperatures (Byrne and Lim 2007). However, these thermometer pills have drawbacks: (1) they cannot be used by all people due to medical contraindications, and (2) they can suffer from inaccuracy when hot or cold fluids are consumed (Wilkinson et al 2008). The difficulty in directly measuring core body temperature in ambulatory settings has led to the search for a practical alternative technique.
One non-invasive approach that has received attention is the zero heat-flux (ZHF) method (Fox et al 1973) where an insulated area of the skin is heated until there is no heat flow. The temperature of the skin is then assumed to be equivalent to deep body temperature. Most of the work on this approach has been in laboratory and clinical settings (Yamakage, Iwasaki and Namiki 2002) with recent work focusing on improving measurement of dynamic temperature changes (Steck, Sparrow and Abraham 2011); decreasing the technique's response time (Teunissen et al 2011); and adapting the ZHF method for use in ambulatory environments (Gunga et al 2008, 2009). In clinical settings these devices have demonstrated good agreement with esophageal measures, while custom sensors developed for ambulatory environments have had varying degrees of success depending on environmental conditions.
Other researchers have used thermoregulatory heat transfer models to estimate core body temperature (Kraning and Gonzalez 1997; Fiala et al 2001; Havenith 2001). These models use an array of input variables that include metabolic rate, environmental parameters, individual characteristics, and clothing parameters (insulation and vapor permeability). In an ambulatory field setting these models suffer from the fact that not all inputs are available all of the time, and measuring or estimating metabolic rate is difficult. Recent work has focused on combining thermoregulatory heat transfer models with metabolic rate estimators that use heart rate with ambient temperature modifiers to account for skin blood flow (Yokota et al 2008). This real-time model provided accurate group-mean core body temperature estimates in a number of different environmental and clothing conditions (Degroot et al 2008). While this method shows promise it still requires many input parameters that must be measured independently from an individual such as environmental conditions and clothing characteristics.